It is well known that glass fibers have exceptional tensile strength. It is also well known that long lengths of pristine fibers rarely possess the tensile strength of short glass fibers. This is because the surface of glass is fragile and susceptible to scratches and contaminants that produce breaks. The number of potential break sites increases with the length of the fiber so the tensile strength is length dependent. (The actual relationship is non-trivial. There is some evidence that the dependence is bimodal).
It has been recognized widely that the strength of a long fiber can be improved by protecting the fiber from surface contaminants and physical abuse. Abuse in this context has been found to mean any physical contact-- such is the fragility of the unprotected surface. It is therefore standard in the art to apply a protective plastic coating to the fiber as it is formed. Considerable effort has been expended in the technology of coating long fibers. Such fibers are of interest mainly for the transmission of intelligent information via light signals, and are referred to as optical fibers.